The invention relates to internal combustion engines and more particularly to an arrangement for varying the area of an exhaust port of a cylinder of such.
The invention relates to "ported" engines rather than "valved" engines and therefore since two-stroke engines more commonly use ports instead of valves the invention will be discussed with reference to a two-stroke engine, although it should be appreciated that the invention should not be considered limited to such.
A major disadvantage of the type of two-stroke engine well-known in the prior art is the loss of fuel/air mixture from the cylinder that occurs during scavenging of the engine. In a standard two-stroke engine the skirt of the piston serves to close the ports in the cylinder, one or more of these ports serving to provide a passage for the injection of a fresh charge of fuel/air mixture to the cylinder and one or more other ports serving to provide an exhaust outlet for the combusted gases. The transfer ports and exhaust ports are arranged in the cylinder so that on downward movement of the piston the exhaust ports are uncovered first, the high pressure differential between the gases in the cylinder and atmospheric pressure causing the combusted gases to flow out of the cylinder into an exhaust passage which leads to an exhaust pipe which delivers the gases to the atmosphere. On further downward motion of the piston the transfer ports are uncovered enabling a fresh charge of pressurised fuel/air mixture to be delivered to the cylinder for combustion. The fuel/air mixture is commonly pressurised in the crankcase of the engine by the downward motion of the piston but could be equally well pressurised by a supercharger or turbocharger. The pressurised delivery of gas also serves to force combusted gases from the cylinder, a process known as scavenging.
The disadvantage of the type of operation described above is that a considerable amount of fuel is unnecessarily exhausted from the cylinder without undergoing combustion. The time during which both the transfer and the outlet ports are uncovered is controlled solely by the motion of the actual piston itself, the only means of closing the apertures being provided by the piston. When the piston moves towards the top of the cylinder it closes first the transfer ports and secondly the exhaust ports. During the period between the closure of the transfer and exhaust ports the piston drives from the cylinder through the exhaust ports some of the fresh charge that has been delivered by the transfer ports.
Further disadvantages of the known two-stroke engine occur since compression of the charge cannot commence until the piston has passed the top of the exhaust port or the top of the uppermost of a plurality of exhaust ports. This is disadvantageous since the peak pressures in the cylinder are limited and hence the thermal efficiency of the engine adversely affected.
Attempts to deal with the above mentioned problems have been disclosed in U.K. application no. GB 2145 467A and also in International Application No. PCT/US88/03136.
In GB 2145 467A there is disclosed an exhaust valve wherein a shutter is arranged to be introduced into an exhaust passage of a two-stroke engine, varying the effective area of the exhaust port of a cylinder of the engine. The effective area of the exhaust port is the area of the port through which gases can pass to the exhaust passage. The exhaust port itself will have a fixed total area since it is an aperture machined in the wall of the cylinder. However the shutter of GB 2145 467A uses a shutter to close the same exhaust port area to reduce the effective area of the port. The motion of the shutter is synchronised with the motion of the piston moving within the cylinder. The shutter begins to reduce the effective area of aperture as the piston commences to uncover the transfer port. The shutter further reduces the effective area of the exhaust port as the piston continues its downward motion and commences its upward motion. The shutter begins to increase the effective area of the exhaust port after the piston has passed the exhaust port and it is sealed by the piston skirt. The arrangement disclosed serves to decrease the amount of fuel/air mixture exhausted from the cylinder before combustion and to increase the peak pressure of the gas in the cylinder. However, the effectiveness of the arrangement is limited since the shutter motion is synchronised with piston motion and cannot be adjusted in response to varying engine parameters such as engine speed and engine load.
In International Application No. PCT/US88/03156 an exhaust valve arrangement is disclosed wherein a shutter is used to reduce the effective area of the exhaust port of a cylinder of a two-stroke engine. The motion of the shutter is substantially synchronised with the motion of the piston in the cylinder. The application discloses a mechanical system whereby the timing of the shutter motion is varied automatically with engine speed, the system acting to introduce a phase difference between the oscillation of the shutter and the oscillation of the piston.